Anthropogenic Nutrient Loading in the Northeastern US 1920-2000

Human activities have dramatically altered biogeochemical cycles on local to global scales. Altered fluxes of nutrients (nitrogen, phosphorus) to freshwater systems have been driven directly by human-mediated fluxes (e.g., industrial N fixation) and indirectly due to changes in land and water systems that alter rates of biogeochemical transformations and transport vectors for nutrients. The Northeastern United States as a region underwent many biophysical and political changes over the 20th century, making it an excellent case study for understanding human-biogeochemical relationships over time. From 1920 to 2000, this region experienced significant losses of agricultural land and increases in forest and urban land cover. Furthermore, major national and state legislation, including nuisance laws and the Clean Water Act, was passed during the 20th century to control pollution problems, and major technological advances in wastewater treatment were made. Our goals were to: 1) describe quantitative changes in the spatial patterns of water quality over time, 2) understand the proximate (e.g., changes in land use, new technology) and 3) ultimate (e.g., major demographic, economic, social shifts) drivers of those patterns. Using data from the historic Census of Agriculture, the National Atmospheric Deposition Program, and primary literature, we create a comprehensive time series database of anthropogenic N and P inputs to the Northeast terrestrial system. Inputs are estimated for each county at decadal time scales. Inputs included atmospheric deposition of nitrogen, fertilizer, manure, enhanced biological nitrogen fixation, and domestic waste. We used this database, in conjunction with data on land use, reservoirs, climate, and stream nutrient loads estimated from USGS NWIS to develop a modified export coefficient model for 26 watersheds in the Northeast. We then used this model to estimate nutrient loads at the decadal scale for all HUC 8 watersheds in our study region. Over a multi-decade timeframe, we find temporal trends in nutrient fluxes at the regional scale as well as a major restructuring of the spatial patterns of these fluxes. Inputs of N and P increased during the first half of the century; however, after the 1960s total inputs of N continued to increase, whereas they decreased for P (primarily due to decreased fertilizer and manure P, and also due to detergent bans). Inputs of N in fertilizer increased, while they decreased for P, indicating a shift in fertilizer composition. Agricultural inputs of N and P dominate (fertilizer, manure, N fixation) regionally, but atmospheric N deposition became increasingly important starting in the 1980s. Regionally, we see a decrease in agricultural land use during the 20th Century, but also a geographic concentration and intensification of agriculture. This led to strong agricultural hotspots of nutrient loading, particularly around the Chesapeake Bay. Agricultural inputs became decreasingly important locally as urban centers expanded and atmospheric deposition of nitrogen increased but remained significant contributors of nitrogen and phosphorus.